ACP: expose undo and session fork/close (btw support)

Motivation

The ACP adapter currently exposes /compact as a built-in slash command, but two adjacent session operations have no ACP surface:

• Undo — Session.undoHistory(count) exists in the node SDK (the TUI's /undo drives it), but ACP clients cannot reach it. Hosts embedding kimi acp (IDEs, chat bots) have no way to roll back turns.
• Fork / btw — KimiHarness.forkSession exists in the node SDK, but ACP clients cannot create an ephemeral fork to run a side conversation that cannot pollute the main session's context.

Proposal

1. /undo [count] built-in slash command (same shape as /compact): parse an optional positive-integer count, refuse while a turn is running (the kernel's context.undo has no active-turn guard, so an undo landing mid-turn would splice the history the in-flight turn is still reading), and call Session.undoHistory(count).
2. kimi/session/fork extension method { sessionId } -> { sessionId }: fork a session into an ephemeral copy and register it as a first-class ACP session the client can session/prompt with the full streaming/tool surface. Kernel-side SESSION_FORK_ACTIVE_TURN rejection is preserved.
3. kimi/session/close extension method { sessionId, archive? }: close and drop a session from the adapter; with archive: true also archive the on-disk directory (the fork cleanup path). Requires an archiveSession passthrough in the node SDK (agent-core already implements it).

Extension methods live in the kimi/* namespace so they cannot collide with future ACP spec methods.

Known limitation

ACP-supplied mcpServers are not carried into forks: resumeSession rebuilds MCP from the on-disk config only. Carrying caller-supplied MCP servers through fork is a follow-up.

Start with the ACP adapter and its existing /compact built-in command, then trace Session.undoHistory and KimiHarness.forkSession in the node SDK. Check how session/prompt and streaming/tool handling register sessions, and where archiveSession would fit. Done means /undo, kimi/session/fork, and kimi/session/close work with the stated guards and archive behavior.